The 515 series here: http://www.hammondmfg.com/pdf/9C-500-519.pdf indicates a 515-0930 model, which is 7.4" wide, 6" front to rear skirt, and 2.13" tall at the back. The "flat" part in the back (where the controls would go) is 2.5" of the total 6" front to back distance, which is ample space for two rows of knobs and 16mm pots, provided the knobs aren't huge. The 7.4" width is ample room for, say, three stomp switches to be mounted. Not sure what you want it for, but that's a sweet little chassis for someone who wanted to make a modest multi-FX unit.

The other thing I'll say in favour of the smaller ones is that they are rigid by virtue of their small size, such that stepping on a stompswitch does not result in "give" in the chassis top, like it might with the much larger sizes. They also give the builder a free choice in whether to mount the jacks along the rear skirt or the side.